Influence of Sintering Temperature on the Marginal Fit and Compressive Strength of Monolithic Zirconia Crowns

Statement of the Problem: Increasing the sintering temperature is suggested by some manufacturers as a way to enhance the translucency of monolithic zirconia crowns. Meanwhile, its effect on the marginal fit and compressive strength of the restoration is not fully understood. Purpose: This study aimed to evaluate the effect of sintering temperature on the marginal fit and compressive strength of monolithic zirconia crowns. Materials and Method: In this in vitro study, thirty crowns of pre-sintered monolithic zirconia were milled and sintered in a special furnace at either 1450°C or 1550°C (n=15 per group). The marginal gaps were measured at 18 spots on the dies with a digital microscope. To evaluate the compressive strength, the specimens were cemented on brass dies by using conventional glass ionomer cement. Vertical load was applied by a universal testing machine until fracture. One-way ANOVA test was used to analyze the results (α=0.05). Results: The marginal gap was not significantly different between the two groups (p= 0.062). A significantly higher mean value of compressive strength was observed in crowns sintered at 1550°c (1988.27±635.09 N) than those sintered at 1450 °c (1514.27±455.11 N) (p= 0.026). Conclusion: Although increasing the sintering temperature would not affect the marginal gap of monolithic zirconia crowns, it could significantly improve the compressive strength of zirconia restorations.


Introduction
Zirconia-based restorations are very popular due to their supreme strength, outstanding fracture resistance and excellent biocompatibility [1][2][3]. Zirconia ceramics exist in multiple crystalloid modes including monoclinic (room temperature up to 1170°C), tetragonal (1170-2370 °C) and cubic (2370°C to melting point). When zirconia cools down to room temperature, phase transformation from tetragonal to monoclinic happens to it, along with a 3%-5% volume expansion, which causes stresses through the material and can led to destruction [4][5].
Being highly opaque, zirconia cores are generally veneered with veneering porcelain. Clinically, the veneer is much more vulnerable to chipping and failure [6]. Aiming to decrease the costs and defeat the chipping problem, a nano-crystalline zirconia have been developed as yttriastabilized tetragonal zirconia polycrystal (Y-TZP), with satisfactory optical and mechanical properties, which allows fabricating fixed dental prostheses of monolithic zirconia without veneering ceramic [2,7].
These restorative solutions have no porcelain overlay material to jeopardize the shear or to cause fracture.
Moreover, they do not require specialized pressing techniques and equipment. Constructing mono-block restorations from zirconia could enhance the mechanical stability and extend the domain of applications. There are sig-nificant advantages for monolithic crowns including reduced fabrication time, cost-effectiveness, and elimination of the core-veneer interface [7][8], as well as more conservative preparation due to the absence of the veneer layer [9]. Nonetheless, the high opacity of such zirconia restorations interferes with their esthetics [10]. Besides changing the formulation, modifying the fabrication and sintering process of zirconia can enhance the translucency and consequently improve the appearance [11]. Each sintering parameter can strongly affect the properties of zirconia [12]. Studies about the relationship between the microstructure and mechanical properties of Y-TZP reported that the transformation toughening depends on the grain size of these ceramics [13][14][15]; which is, in turn, influenced by any alteration in sintering time and temperature [16].
Sintering temperature generally spans from 1400°c to 1600°c based on the manufacturer's instructions. Some manufacturers suggest increasing the sintering temperature as a way to enhance the translucency of monolithic zirconia crowns. Higher sintering temperature and time results in larger grain size, which is more likely to experience stress-induced transformation to a balanced structure and consequently increase the material toughness. The maximum toughness is achieved close to 1µm grain size.
Beyond this critical threshold, the material transforms from tetragonal to monoclinic phase spontaneously, and consequently, diminishes in stability [17]. Sintering temperature highly determines the grain densification and thus the mechanical properties of specimen [18]. It may also affect the marginal fit, since ceramics shrink toward their bulk to different extents when cooling from higher temperature to room temperature [19].
Several studies have inspected the effects of changing sintering parameters (temperature and time) on the grain size, translucency, and biaxial flexural strength of zirconia materials [11][12][18][19]; however, limited evidence exists about the effect of sintering temperature on the compressive strength and marginal discrepancy of monolithic zirconium [20][21]. Thus, the current investi-    (Figures 3 and 4).    The mean marginal gap of the monolithic zirconia group sintered at 1450•c was higher than those sintered at 1550•c; however, the difference was not significant ous study [22]. The minimum force leading to fracture was recorded by the computer software system that con- were assessed, and one-way ANOVA test was applied to analyze the results (α=0.05).

Results
The mean marginal misfit of the monolithic zirconia crowns sintered at 1450°C was 51.35±4.33μm and 48.
18±4.60μm for those sintered at 1550°C, which was not significantly different between the two groups (p=
In the present study, the mean marginal misfit of monolithic zirconia was within clinically acceptable ranges for both groups of sintering temperatures. Moreover, the marginal gap was measured without cementing the crowns to preclude the variety of cementation procedure (applied force and the mixture viscosity) [26]. Numerous investigations demonstrated that cementation significantly increased the marginal misfit, depending on the luting agent [27][28].
The marginal discrepancy of restorations has been assessed with various methods, the most common of which are direct microscopic view and the replica method [29][30]. As a non-destructive straightforward tech-nique, direct viewing evaluates the marginal fit by means of stereomicroscopes [31] and optical microscopes [26] along with an image analysis software, as used in the present study.
The current study also detected a direct relationship between the sintering temperature and compressive strength of monolithic zirconia crowns. Despite the narrow temperature range (100°C), the compressive strength significantly improved, substantiating the fact that even small sintering temperature variations can significantly affect the compressive strength of zirconia crowns.
In line with the present study, Ersoy et al. [18] noticed improved flexural strength of zirconia, fine structure, and densification following the concurrent increase of sintering temperature and decrease of sintering time.
Examination of the crystal structure showed that all samples were totally sintered to the tetragonal stage and no conversion to monoclinic stage was distinguished.
Similarly, Stawarczyk et al. [12] studied the influence of sintering temperature on the contrast ratio and biaxial flexural strength of zirconia discs. They achieved the highest flexural strength between 1400°C and 1550°C; however, Stawarczyk et al. [12] reported that the flexural strength declined over 1550°C due to immigration of yttrium to the grain boundaries. Tekeli and Erdogan [32] documented that higher sintering temperature and Transformation toughening in zirconia ceramics depends on the grain size [13][14][15], which is, in turn, affected by the sintering temperature [16]. Although higher sintering temperature creates zirconia specimens with larger grains and consequently higher toughness, there is a threshold for the grain sizes beyond which tetragonal-monoclinic transformation and subsequently decrease of material stability is expected [17]. It seems that 1550°C is the critical sintering temperature; beyond which grain size larger than the critical size is obtained [5,[33][34]. It was stated that larger grain size might enhance crack formation [35] and consequently decrease the mechanical strength of material. However, such a trend was not observed in the present study as the temperature was below the critical threshold.
The failure load in complete crowns was reported to range from 980 to 1400 N [36][37]; while in the present study, the mean compressive strength of monolithic zirconia was 1514N (at 1450°c) and 1988N (at 1550°c).
The present study assessed the compressive strength

Conclusion
Considering the restrictions of this in vitro study, it can be driven that increasing the sintering temperature would not considerably influence the marginal gap of monolithic zirconia crowns. However, it can significantly improve their compressive strength.